1. Field of the Invention
The present invention relates to radio frequency couplers.
2. Description of the Prior Art
Heretofore, couplers for radio frequency signals, in particular output couplers for radio frequency oscillators such as pencil tube oscillators and the like, include a coaxial transmission line terminating in an inductive loop disposed in the oscillator cavity. This structure takes several forms. In one form a coaxial connector has its center conductor extending beyond the center conductor into the cavity of the oscillator and terminating in the oscillator in an inductive loop. The connector, the loop and the cavity are disposed in fixed relationship to form a loop having a fixed orientation and a fixed inductive coupling magnitude.
A single orientation coupling loop structure having a single coupling magnitude is not completely satisfactory in a mass production and mass end use environment. Fixed loop coupling cannot be adjusted to compensate for variations in production variables such as exist in tubes, cavity dimensions, etc. In this situation those oscillators that do not meet minimum specification requirements are discarded, which can be costly. In the mass production end use environment, fixed coupling limits the range of loads to which the oscillator can be coupled.
To overcome these problems has been difficult. In one embodiment, the prior art includes a coupling device wherein the loop is completely secured to a rotatable coaxial connector which is then secured to the oscillator cavity wall. As the connector is rotated in and out of the cavity, the magnitude of the loop disposed within the cavity is altered. The magnitude of the loop is defined by the area circumscribed by the loop, the loop lying in and defining a plane. One difficulty with this technique is that the rotation of the loop causes misorientation of the loop with respect to other elements within the cavity. The loop could in fact be adjusted too close to a high voltage element within the cavity with the resulting voltage arc causing damage to the oscillator or other systems. While it is still possible to provide optimum coupling by this technique for some situations, it is well known that large misorientations of the plane of the coupling loop within the RF circuit will cause a wide variation of loading on that circuit. In this case, if care is not taken, the wide variation in loading could cause damage to the oscillator. However, other techniques may be devised so that orientation of the loop is known and controlled. One underlying factor is assumed for variable loop coupling; that is, the loop is large enough to couple desired RF power from an oscillator that is capable of delivering that power. Great difficulty occurs meeting all of the requirements of a loop with sufficient area to couple desired RF power; i.e., small enough to allow sufficient spacing for high voltage hold off; mechanically stable for temperaure, shock, and vibration requirements; and mechanically adjustable.